The present invention relates to a device for the precision measurement of internal diameters of thin-walled pipes.
The measurement of the internal diameter of such pipes poses problems in that their shape almost invariably deviates from an ideal circle and exhibits a certain amount of ovality as a result of internal stresses set-up in manufacture, of hardening warp or of sheer weight. Conventional measuring means will then fail to give precise measurements of the internal diameter.
It has been previously contemplated to obtain precise measurements of internal diameters with the aid of roundness-flatness measuring machines used to scan the imperfectly round contour of the pipe, after which a computer is used for precise definition of the length of scanned contour which is then transformed into a circle of like length of arc and exact roundness, which is ultimately compared with a master ring representing an ideally round circle of accurately defined diameter. While this known method gives great precision, it necessitates a very costly and complex measuring machine as well as a considerable amount of time in the determination of the internal pipe diameter. This makes this known method ill-suited for production pipe inspection.
In a broad aspect, the present invention provides a device for accurately and rapidly determining the internal diameter of thin-walled pipes in the rough environment of production and production inspection. The device requires little cost and, for ease of changing locations, little weight.
The present invention more particularly contemplates providing a device characterized by an approximately cylindrical body divided into at least three jaw segments and having a diameter slightly less than the internal diameter to be measured, where the jaw segments centrically extend apart to form radially open circumferential gaps between them.
For precision measurement of the internal diameter of thin-walled pipes, the cylindrical body is inserted in the pipe and the jaw segments are then centrically extended apart. The thin-walled pipe is circularly clamped in the process along substantially the entire circumference of the inside wall of the pipe. The amount of extension of the jaw segments is measured on a scale, with a previously determined zero point for reference, for accurate evidence of the internal diameter of the thin-walled pipe. The travel of the jaw segments in centrical extension is minimal and the interruption caused by the radial gaps in the circular contour of the device is acceptable in that any error it produces in the arc length of the circle is minimal and any error in the diameter measured is even less.
The advantages provided by the device arranged in accordance with this invention are seen to lie in its ease of operation which minimizes the risk of error and which more importantly substantially reduces the measuring time associated with the more complex measuring machines discussed above.
Further merits of the device of the present invention are that its relatively small size and weight will not tie it to a permanent location. Ultimately, the price of the device is considerably less than that of presently used measuring machines.
In a further aspect of this invention particularly preferred embodiments of the device are formed by commercial three-point inside micrometer on the feeler points of which are seated jaw segments formed by radially slitting a cylindrical body of the nominal diameter of the pipe to be measured. A key advantage offered by this arrangement of the device is that it is built around a high-grade commercial measuring unit which is fitted with separable attachments to keep the cost of manufacture of the inventive device at a particularly reasonable level. German DAS 1,303,739 shows a micrometer of the type contemplated for use with the present invention.
The jaw segments are made from a cylindrical body of the nominal diameter of the pipe to be measured to make the arched outer surface of the jaw segment match the nominal diameter and to produce gaps, by the width of cut when slitting the cylindrical body radially, which enables the resulting jaw segments to be drawn together over a short distance and the diameter so to be reduced for seating the measuring device, with the jaws closed, in the pipe to be measured.
In further preferred embodiments of the present invention, the jaw segments exhibit a stop face projection radially beyond the arched outer contour. The presence of these stop faces makes for an exactly coaxial fit of the measuring device in the pipe and prevents the device from being cocked and then giving erroneous diameter readings.
In further preferred embodiments of the present invention, the device exhibits various arched outer surfaces on the segments in a stepped arrangement to widen the measuring range. This enables the same measuring unit or the same jaw segments to be used for precision measurements on pipes of various nominal diameters.
In a still further aspect of preferred embodiments of the present invention, the arched outer surfaces of the jaw segments exhibit a length, measured axially from the stop faces, of minimally one-tenth of the internal diameter to be measured. This provides an advantage in that a certain minimum extent of the outer surfaces of the jaws axially prevents the end of the pipe from being slightly expanded by the extending jaws and so giving an erroneous reading.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.